Various methods have been developed for growing crystalline bodies from a melt. One such method is known as the Edge-defined, Film-fed Growth technique (also commonly called the EFG Process). Details of the EFG Process are described and illustrated in U.S. Pat. No. 3,591,348 issued July 6, 1971 to Harold E. LaBelle, Jr. for "Method Of Growing Crystalline Materials", and in U.S. Pat. No. 3,687,633 issued Aug. 29, 1972 to Harold E. LaBelle, Jr. et al. for "Apparatus For Growing Crystalline Bodies From The Melt".
In the EFG Process, a capillary-forming die member is placed in association with a melt of liquid source material so that a growth face on the die member is wetted with a liquid film of source material from the melt by capillary action. A product crystalline body is then grown by first introducing a seed crystal to the liquid film of source material so that crystal formation is initiated, and then drawing the seed crystal away from the growth face at a controlled rate so that the liquid film of source material remains sandwiched between the growing crystalline body and the growth face of the die member. Since the liquid film of source material on the die's growth face is continuously replenished from the melt via the die's capillary (or capillaries), continuous crystalline bodies of significant size may be grown from the melt.
One consequence of the foregoing process is that the liquid source material in the melt is consumed during the crystal growing operation. Accordingly, unless melt replenishment can be effected while crystal growth is under way, the crystal growing operation must be shut down after a period of time to permit the depleted melt to be replenished with additional source material.
Unfortunately, shutting down the crystal growing operation from time to time to replenish the melt raises certain problems. More particularly, shutting down (and starting up) an EFG crystal growing operation is time-consuming and expensive. In addition, the maximum length of continuous crystal which can be grown in a given system is limited in an absolute sense by the frequency with which the crystal growing operation must be shut down to replenish the melt. Furthermore, starting up and shutting down a crystal growing operation requires the crystal growing system to settle into or depart from the optimum crystal growing conditions. During this period of system adjustment, the crystal grown may be of inferior quality to that normally produced.
For these and other reasons, it is preferred that replenishment shutdowns occur as infrequently as possible. Unfortunately, attempts to reduce the frequency of replenishment shutdowns by simply increasing the size of the crucible which holds the melt (and hence the maximum quantity of source material contained in the melt at startup) have been hampered by design considerations for adjacent equipment components.
Attempts have also been made to effect melt replenishment while crystal growth is under way. Such "growth-time" replenishments offer the additional advantage that they may keep the melt level constant, or at least somewhat more constant, during crystal growth. This is important, since it facilitates growing high quality crystalline bodies. Unfortunately, such attempts at effecting melt replenishment while crystal growth is under way have not been fully satisfactory for one or more reasons, including excessive perturbation of the melt as additional source material enters the crucible, lack of reliability, excessive size, high cost, and/or complication of the design of adjacent equipment components.